The invention relates to a method and an apparatus for performing the method of changing the mass ratio of the fuel-air mixture delivered to an internal combustion engine (.lambda.-control). The regulation takes place with the aid of an oxygen sensor whose output potential is fed to an integrating controller which adjusts the metered out fuel quantity.
In known systems of this type, the mass ratio, i.e., the air number .lambda. of a fuel-air mixture fed to an internal combustion engine is changed in dependence on the composition of the exhaust gas. In the known system, a .lambda. sensor or oxygen sensor is exposed to the exhaust gas stream and generates an output voltage in dependence on the exhaust gas composition; its time behavior and effects will be discussed further below. The output voltage of the .lambda. sensor is fed to a controller, preferably an integrating controller, which causes an increase or decrease of the instantaneous fuel fed to the engine as a function of the output signal from the sensor. A change in the air number of the fuel-air mixture in the manner described above can be performed in engines having carburetors as well as those having fuel injection systems although the latter are normally better able to meter out the fuel quantity with precision. In such a control system, the engine itself is part of the control loop so that the dead time of the control loop will be that of the engine throughput time T.sub.t which is a quantity that changes constantly, especially as a function of the engine speed, i.e., rpm.
A parameter of considerable importance in any control loop which uses the output potential of an oxygen sensor is the characteristic of that sensor which is illustrated schematically in FIG. 1 and which, once properly warmed up, exhibits two different switching states. The first of these switching states corresponds to an output voltage of approximately, for example, 900 mV and takes place when the fuel-air mixture to which the sensor is exposed in the exhaust system is rich, while the second output potential is approximately 100 mV and is experienced when the fuel-air mixture is originally lean. The transition between these two sensor potentials is very abrupt and occurs substantially when the air number .lambda. has the value .lambda.=1. In a practical exemplary embodiment, the change between the two states takes place in finite time, however the characteristic curve at the value .lambda.=1 permits regulation to very lightly enriched air numbers only if a sufficiently high threshold value is given. In addition to this disadvantage, operation on the bent part of the curve which is less steep than the other portions of the curve has the further disadvantage that this portion of the curve is temperature-dependent and subject to the effects of ageing. A substantially stable characteristic point of the curve is encountered in presently available sensors at a sensor output voltage U.sub.S of approximately 300 to 350 mV as indicated by the point P in FIG. 1. However, if it is intended actually to use the point P of the sensor curve, then one is forced to operate at a particular value of the air number .lambda.. On the other hand, it is desirable to include the possibility of varying the domain of operation by at least plus or minus 5 percent around the value .lambda.=1 so that the engine operation may be freely chosen to take place approximately between .lambda.=0.95 up to .lambda.=1.05.